V11.2&3, Spring&Fall1985
Informal Logic
Unstated Premises
MICHAEL BURKE
University of Indiana at Indianapolis
Introduction
Many arguments rest on unstated
assumptions. These include assumptions which underlie stated premises
and assumptions which underlie inferences from stated premises to conclusions. Consider the argument below.
This wine can't be Port. If it were, it
would be sweet.
That Port is sweet might be an unstated assumption underlying the
stated premise. That this wine is not
sweet might be an unstated assumption underlying the inference. In Robert
Ennis' suggestive terminology, the
first is a "back-up," the second a
"gap-filler." Having now acknowledged back-ups, I will hereafter ignore
them. When speaking of "unstated
assumptions" or "unstated premises,"
I will always mean the gap-filling
variety.
When we set out to identify an argument's unstated premises, we should
be clear whether our purpose is (1)
to identify the argument intended by
the arguer or (2) to determine whether
the stated premises provide the makings of an acceptable argument for
the conclusion .[1] Although this distinction is now frequently noted, few
of those who provide "criteria for unstated premises" bother to provide
two sets. And few provide separate
discussions of what really are quite
different enterprises.
One who does is Robert Ennis
[po 62], who distinguishes "needed
assumptions" ("propositions that are
needed to support the conclusion, to
make the argument a good one ... ")
and "used assumptions" ("unstated
reasons that a person actually used
consciously (or subconsciously, if you
believe in subconscious reasons) as
a basis of argument..."). That a certain assumption was used, Ennis
argues, is best understood as an explanatory hypothesis. The argumentative utterance constitutes an explanandum. And we should attribute to the
arguer whatever unstated assumption
best explains, given what we know
about the arguer and the context,
the details of the utterance. Ennis'
criteria for used assumptions are
simply an application of the general
criteria for evaluating singular explanatory hypotheses.
As best I can judge, Ennis' treatment
of used assumption is successful, so
far as it goes, and a helpful contribution to the subject of unstated premises. Less satisfactory, in my opinion,
are his and other writers' accounts of
"needed" assumptions, and it is on
these that I will focus throughout
this paper. More precisely, I will be
concerned with criteria for identifying
unstated premises when our purpose
is to determine whether stated premises provide the basis for an acceptable argument for a given conclusion.
After explaining my dissatisfactions
with the accounts of Ennis, Hitchcock,
Schwartz and Scriven, I wi II offer an
alternative. First I will note (but not
try to make precise) two criteria which
are uncontroversial.
Uncontroversial Criteria: Sufficiency
and Preservation
Most writers recommend that we
identify as an argument's unstated
premises propositions that would combine with the stated premises to make
the argument deductively valid. This
may reflect not a general commitment
to deductivism, but simply the tradition
108
Michael Burke
of discussing unstated premises in
arguconnection with i i deductive"
ments. Some writers, however, require
only that the unstated premises
strengthen the inference, that the conclusion be more probable relative to
the stated premises plus the unstated
ones than relative to the stated premises alone. The stronger requirement
might be defended by arguing, as
Thomas Schwartz does [1981, p. 245],
that ,/ any argument can be construed
as deductive without impairment."
But what is uncontroversial is what I
will call the "sufficiency requirement": The unstated premises must
be sufficient to validate, or at least to
strengthen, the inference.
But now for any deductively invalid
inference there are infinitely many sets
of propositions whose addition would
validate or strengthen the inference.
How are we to choose? One justly
popular criterion is what I will call
"preservation," according to which we
should try to preserve the role of the
stated premises. So long as our purpose
is to evaluate the strongest argument
that can be based on these premises,
we will not want to add propositions
that would nullify their contribution
to the strength of the inference. Otherwise the resulting argument would not
be based on the stated premises. But
even when we combine sufficiency
with preservation, we will always
still be left with an infinite number of
candidates. (Disjoining the negation of
the conjunction of the stated premises
with any statement that entails the
conclusion will produce such a candidate.)
Section I: Ennis
To the criteria of sufficiency and preservation (for which he uses other
names), Ennis adds IIfidelity" and
plausibil ity."
Ennis' fidelity is not the criterion
familiar in discussions of used assumptions, i.e., fidelity to the beliefs and
intentions of the arguer, but what
Ennis calls IIfidelity to the argument."
Ji
And as is plain from Ennis' brief
explanation [po 70], fidelity to the argument is not so much a criterion for
unstated assumptions as an injunction (to apply when the argument,
rather than the conclusion, is lithe
focus") against revision of the stated
parts of the argument. It will not be
relevant to our concerns.
Ennis' fourth criterion is plausibility.
The essentials of his explanation are
contained in these passages [po 71]:
The plausibility criterion calls for the
assumption identifier to be as generous
as possible. One does this (selecting
from the possible gap fillers) by attributing to the argument or conclusion
maximally-plausible propositions. The
reason for this is that it is not fair
to say that a less plausible proposition
is needed by a position, when a more
plausible one is available to do the job
of gap filling ... Since there is ... sometimes a choice between attributing one
broad one and two more-narrow ones,
the criterion, more fully stated, calls
for attributing assumptions with a
maximal joint plausibility.
There is a problem. Initially to oversimplify, the problem is that the most
plausible candidate will always be the
weakest candidate. And if 'P' abbreviates the conjunction of the premises
of some deductively invalid argument,
and 'e' its conclusion, the weakest
validating proposition is expressed
simply by "if P then C'. (Note that this
candidate will violate neither preservation nor Ennis' fidelity.) I will call this
the I I reiterative" candidate, since
several writers remark that it merely
"reiterates" the arguer's claim that
the conclusion follows from the premises. Although the reasons differ,
and are not always clear (I'll give
mine later), few writers think it helpful to add this "logical minimum."
More precisely, few, if any, think it
generally to be the best choice.
Oddly, Ennis takes no notice of this
problem, even though he does mention
reiterative tacit premises. Ennis notes
that he differs with Scriven in holding
that these minimum links sometimes
are a useful addition, but he doesn't
Unstated Premises
explain how, on his criteria, we ever
could justify a preference for one of
their validating competitors.
But now am I right in saying that
the weaker of two claims cannot be the
less plausible? After three points of
clarification, I believe the reader will
be ready to agree.
First, I am using 'weaker' in its
standard logical sense: P is weaker
than Q if and only if Q entails, but is
not entailed by, P. Ralph Johnson
[p. 7] suggests that on occasion the
"stronger" candidate may be the more
plausible, but he is using' stronger' in
a different sense. Comparing 'Most
redheads are quick-tempered' with
'Most redheaded women are quicktempered', Johnson finds the former
to be stronger, in an "as-yet unanalyzed" sense, and also more plausible.
But since neither statement entails
the other, neither is stronger in the
standard logical sense. Johnson does
not suggest that in this sense of
'stronger', the stronger of two claims
might be the more plausible.
Second, I am not denying that it may
often be more plausible to hold that an
arguer had in mind the stronger
claim. With reference to the same
example discussed (later) by Johnson,
Scriven [po 166] says that "it's a little
more plausible to suppose that the
arguer
believes
that
red-headed
people are bad-tempered than that he
or she bel ieves that red-headed women
are bad-tempered ... Perhaps so. But,
of course, its being more plausible that
a certain assumption was made is
quite different from its being more
plausible that that assumption is true.
Third, my claim that the weaker of
two propositions is never the less plausible is meant to carry the implicit
qualification: providing its relative
weakness is realized. The entailments
of a proposition sometimes are less
plausible to us than the proposition itself, until we notice that they are entailments. That a purple shoe can be evidence that all ravens are black is wildly
implausible-initially, but it is a logical
consequence of three principles each of
which is extremely plausible-initially.
/I
109
Once we are acquainted with the raven
paradox we revise our assessments. We
consider the (conjunction of) the principles no more plausible than their surprising consequence. Now when deciding which of two assumptions to identify as the assumption needed by an argument, should we select (other things
being equal) the assumption that had
the greatest initial plausibility-or the
one we find most plausible when our
reflections have concluded? Given that
Ennis says, "The plausibility criterion
calls for the assumption identifier to be
as generous as possible ... The idea is to
give the argument or conclusion its best
chance to succeed ... " [po 71], he could
hardly suggest that we always select
the candidate with maximum initial
plausibility. Sometimes we find decisive objections to that candidate while
discovering strong support for one of
the others. But if we are to choose on
the basis of final plausibility, then since
we realize that the reiterative candidate
is entailed by any of its validating competitors, the plausibility criterion will
always be powerless to justify our preference for one of those competitors.
Ennis seems tacitly and inadvertently
to concede the point when he introduces the one reiterative candidate he
discusses by saying [po 75], "But I can
note a minimal coverage maximallyplausible claim that would serve as a
piece in a reconstructed version of the
Will argument.
The only apparent
ground for regarding the introduced
claim as "maximally-plausible" is precisely its "minimal coverage." Subsequently, Ennis remarks [po 75], "Since
we are not operating in a real context, I
cannot suggest whether the request
should be to consider only the specified
claim or a more general one," He
seems not to have thought to ask how
his criteria could warrant a preference
for the more general claim,
In sum: Since it is not apparent how
plausibility or any of Ennis' other criteria can enable us ever to reject the reiterative candidate in favor of one of its
stronger competitors, and since, as we
will see, there are good reasons for
thinking such competitors sometimes to
/I
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Michael Burke
be preferable, I conclude that Ennis'
account is inadequate.
Section II: Hitchcock
One of the more active contributors
to the theory and pedagogy of informal
logic is David Hitchcock, whose generally excellent textbook includes a relatively thorough treatment of tacit premises. The policy he recommends is to
add the weakest candidate satisfying
validation, preservation! and "testability.1f His strategy for blocking the
reiterative candidate is contained in
these passages:
Any argument can be given a sound
inference by adding as a tacit premise
that if the explicit premises are true, the
conclusion is true. In general, however,
such a tacit premise is not testable ...
The rationale for the criterion of testability is that in supplying a set of tacit
premises for an argument we are trying
to see whether a good case can be made
for the conclusion. If we put in a statement that is not testable, then, when we
get to the stage of assessing whether we
are warranted in believing the statement, we have to think about how it
could be shown to be true (or probably
true). But unless there is some way of
testing it, this is impossible.
Consider the argument" He's a homosexual, so he shouldn't be appointed to
this politically sensitive post." The
weakest tacit premise which will make
the conclusion follow is the conditional
statement: "If he's a homosexual, he
shouldn't be appointed to this politically
sensitive post." But this statement is not
testable; it merely reiterates the claim of
the argument that it follows from his
being a homosexual that he shouldn't be
appointed. We need a more general
statement; for example, "Homosexuals
ought not to be appointed to politically
sensitive posts." This is a principle; one
could ask for its justification (a justification which would presumably point to
some link between homosexuality and
political sensitivity) and also argue
against it (on the ground, for example,
that sexual orientation should be irrelevant to hiring decisions and that a
responsible political organization should
show leadership in this respect). (p. 83J
Hitchcock does not give a definition of
'testable!. He is content to say that testability may be either "direct or indirect" and that it need not be empirical. The testing may be "in terms of
our usage of a certain word ... of a
mathematic theory ... of a normative
theory of ethics or aesthetics ... /I[p. 83].
Now I think we can accept both the
criterion of testability and the rationale
offered for it. But will this criterion exclude reiterative tacit premises? Will it,
for example, exclude the statement, "If
he's a homosexual, he shouldn't be appointed to this politically sensitive
post,'! call it'S', that Hitchcock uses it
to exclude?
It is clear, I believe, that the answers
are "no./I But lees look at Hitchcock's
reasons for saying otherwise. First, he
notes that S merely reiterates the
claim of the argument that it follows
from his being a homosexual that he
shouldn't be appointed." That's true,
but what reason is there to think the
latter
claim
untestable?
Second,
Hitchcock contrasts S with the statement, call it 'P', "Homosexuals ought
not to be appointed to politically sensitive posts./I Hitchcock says of P that
"one could ask for its justification ...
and also argue against it./I But one
could ask for the justification of S. In
general, considerations which support
P will support S. One could also argue
against S. One could use the premises
Hitchcock suggests for arguing against
P.
To see the futility of Hitchcock's approach, consider this. Even if S is
understood as a material conditional, S
will be sufficient to validate Hitchcock's
argument. Now if S is so construed, it
will be equivalent to "Either he's not a
homosexual or he shouldn't be appointed to this politically sensitive
post." And it seems safe to say that this
(material) disjunction is untestable only
if at least one of its disjuncts is untestable. So if S is untestable, and therefore
ineligible ever to serve as an unstated
premise, so is at least one of these disjuncts. Furthermore, if S and one of the
disjuncts are untestable, they would
also be unsatisfactory as stated premII
Unstated Premises
ises or as conclusions. These consequences are unacceptable.
More generally, one would think that
a given reiterative tacit premise is testable if the inference which it validates
is testable. If so, then if reiterative tacit
premises are, "in general," untestable, so are formally invalid inferences.
In light of the above, it is plain that
Hitchcock's policy-to add the weakest
candidate satisfying validation, preservation, and testability-generally will
select the reiterative candidate. In
seeking to exclude this candidate by
reference to an intrinsic defect such as
untestability, Hitchcock overlooks the
fact that a statement which is unsuitable to serve as one argument's tacit
premise may serve quite satisfactorily
as another's conclusion or explicit premise (or even as another argument's
tacit premise). What Hitchcock could
have said is that argument evaluation is
never facilitated by adding the reiterative statement, because it is never
easier to test this statement than to
test the original inference. (Of course,
it doesn't follow that we would be
justified in adding some stronger statement. All that follows is that we might
as well add nothing. Hitchcock would
still need to provide and justify additional criteria.)
Section III: Schwartz
One of the very few writers with a criterion capable of blocking the reiterative candidate is Thomas Schwartz. To
the familiar criteria of validation, fidelity (which he says to be important only
when the aim is to identify the argument intented by the arguer), and plausibility (which he calls "Generosity"),
Schwartz adds "Generality": "Add
premises that are as general as possible, consistent with Fidelity and Generosity" [1981, p. 236]. The rationale for
this criterion is explained thus:
Say your are deciding which of two [validating] candidate premises to add to an
argument, and neither is preferable according to Fidelity or Generosity [plausibility], but one is more general than
111
the other. The reason you should choose
the more general candidate is that it is
a likely reason or justification for the less
general one, hence a likelier ultimate
reason or justification for the argument's
conclusion-a likelier premise, in other
words [1981, p. 236].
What we would need to understand l
before we could accept Schwartz's account, is whYI when the purpose in
identifying unstated premises is to make
the argument as strong as possible,
generality should be a desideratum. Ordinarily, an argument's strength is
thought to depend on the plausibility of
its premises, on the strength of the link
between its premises and conclusion,
and on the absence of circularity. Neither of the first two factors can provide
a rationale for the generality criterion.
This criterion is to apply only when
plausibility and validation yield no decision. And there is no apparent way in
which the criterion can be justified by
reference to the need to avoid circularity.
Schwartz! s rationale, surely unaceptable, is based on the idea that only an
"ultimate reason'! (basic proposition?)
is a genuine premise. This idea is supported by neither the ordinary, the legal, nor the logical usage of "premise," and it is inconsistent with most
of Schwartz's own examples of stated
and tacit premises. It also conflicts with
Schwartz's statement, found on the following page [1981, p. 237], "Since any
statement about any subject could be
premise of an argument, you would be
omniscient if you could tell the truth or
falsity of every premise of every argument you might ever come across."
And even if we agreed to the narrow
definition, this would merely shift the
question to why an argument's undefended parts must be limited to premises so defined.
I suspect that the true appeal for
Schwartz of the generality criterion is
that he thinks it will facilitate (fair)
critici sm. For stated prem i ses, Schwartz
proposes a "test of Sufficient Generalitv. "
Is there some natural way to make the
premise in question more genera!...?
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Michael Burke
If SO, .•. is there some relevant difference
between the cases to which the premise
already is applied and the others in the
wider class-some difference that would
justify applying the premise to the former cases only? If not, the premise fails
the Sufficient Generality test... the author is committed to a more general
premise ... The test... is useful as a critical tool because the more general of two
statements often is the easier to criticize ... one cannot immunize one's premises from counter-examples by restricting their generality unless there is a
plausible reason for the restriction. So if
there is no plausible reason not to generalize a premise a certain way, a counterexample to the generalized version refutes the original. [1980, p. 188]
Without commenting directly on the
merit of these ideas for deal i ng with
stated premises, I wish to ask whether
they can justify a generality criterion
for unstated premises. Suppose that A
and B are the leading validating candidates, that B is more general than A,
that we see no reason for the restriction
contained in A, and that we consider B
to be about as plausible as A. Now why
should we choose B? Suppose that
in accordance with the generality criterion, we proceed to identify B as the
tacit premise. And suppose that subsequently we discover a counterexample
to B (one which is not a counterexample to A). Suppose, finally, that
we conti n ue to see no reason for the
restriction contained in A (except for its
ability to block the counterexample).
Now what should we do? Should we replace B with A? Or can we properly
conclude our analysis, charging that the
argument depends on an assumption
shown to be false? (If we can, then just
criticism has indeed been facilitated by
use of the generality criterion.) Evidently Schwartz would have us retain
B. Schwartz says that "one cannot immunize one's premises from counterexamples unless there is a plausible
reason for the restriction." (I take it
that he means "unless one has a plausible reason for the restriction .")
But this is inconsistent with the plausibility criterion, which is supposed to
take precedence over the generality cri-
terion. If we are now able to refute B,
then B is now significantly less plausible than A-unless we are able also to
refute A. And to refute A it would not
be sufficient to point to the refutation
of B plus the absence of any apparent
reason for the restriction contained in
A. It would be necessary to show that
no such reason exists, i.e., that A is
false if B is. Short of that, our final assessment should be that the argument
depends on a tacit premise, A, which
there is no apparent reason to accept.
That accords better with our purpose
(evaluating the strongest argument that
can be based on the stated premises)
than concluding that the argument depends on a tacit premise, B, of demonstrable falsity. The plausibility criterion
will not permit us to continue to identify
B as the tacit premise when we have
come to have a stronger reason for
rejecting B than for rejecting A. The
"immunization" not only would be permissible; it would be obligatory. So
how, consistently with the plausibility criterion, could just criticism ever
be facilitated by use of the generality
criterion? If there is an answer
Schwartz has not provided it.
f
To sum up: Although Schwartz's
generality criterion has the (presumed)
merit of frequently rejecting the reiterative candidate, its rationale is obscure.
There is no apparent way to justify it
by reference to the aim of making the
argument under analysis maximally
strong. No candidate can have greater
(final) plausibility than the reiterative
candidate. Schwartz's stated rationale-that the more general candidate is
a likelier ultimate reason and, therefore, a likelier premise-depends on
the unacceptable notion that only ultimate reasons are genuine premises.
Finally, it seems futile to seek to justify
the criterion by reference to the aim of
facilitating just criticism. If we should
ever come to have a stronger reason for
rejecting the more general candidate
than for rejecting the less general one
the plausibility criterion would require
that we then designate the less general
candidate as the tacit premise.
I
Unstated Premises
Section IV: Scriven
Michael Scriven's highly-regarded
textbook Reasoning contains unusually
thorough coverage of what Scriven calls
"unstated assumptions" or "missing
premises. It is not easy, however, to
extract a coherent account.
II
On the one hand, Scriven wants us to
look for an unstated link that is "plausible" [pp. 174-175], "true" [po 174],
"obviously" assumed [po 83] and "persuasive or at least not objectionable in
and of itself" [po 173].
On the other hand, Scriven says,
"Your task." is to call attention to ...
the important, the debatable, the hazardous, and the hidden points... In
short, you're searching for a weak
link ... " [po 164].
Scriven notes that there is
some
tension" among his various desiderata.
But between "obviously assumed" and
"hidden," and between "a weak link"
and "not objectionable in and of itself," tension has escalated into outright contrariety.
1/
Part of the confusion, as I see it,
stems from Scriven's failure to give
separate accounts of used assumptions
and what he calls "required" assumptions [po 81]-or else to limit his account to one or the other. Scriven
writes:
You also want to try to relate the assumptions as you formulate them to
what the arguer would be likely to know
or would believe to be true. The principal
function of argument analysis is not that
of reconstructing the state of mind or
body of beliefs of the arguer, but this
may nevertheless be a relevant consideration in some cases. You need to
decide whether you are arguing against
the arguer or the argument. [po 85]
Regrettably, Scriven does not go on to
provide separate accounts of "arguing
against the arguer" and "arguing
against the argument."
Also contributing to confusion, I
believe, is a failure to keep separate
113
two questions that Scriven himself
distinguishes: (1) What unstated propositions does an argument (or arguer)
assume? (2) Which of an argument's
(or arguer's) unstated assumptions are
worth identifying as such? Most discussions of enthymematic arguments
focus on the first question. Since
Scriven has (as we will see) a rather
broad conception of "unstated assumptions," he understandably gives much
attention to the second. Indeed, the
second of his two sections on unstated
assumptions is titled "Significant
and Insignificant Assumptions." The
significant assumptions, those worth
identifying, are the "important,"
"illuminating," "debatable," "vulnerable" "hazardous" or "hidden"
ones. I ~uspect that Sc;iven's other criteria ("plausible," "not objectionable," "obviously assumed") are given
with the first question in mind, especially as concerns used assumptions.
But Scriven never raises the first question without proceeding to conflate
it with the second [ct. pp. 85-86], and
it is not obvious how he would answer
it.[2]
The value in Scriven's treatment is
found, I believe, in his illustrations,
the most extended of which concerns
the argument below.
He is a homosexual.
Hence, he is prone to blackmail.
Hence, he is a security risk.
And, this job requires security clearance.
So, he should not be apPOinted.
Scriven begins by rejecting as "at too
trivial a level" [po 169] candidates like
"Homosexuals are prone to blackmail" and "Male homosexuals are
prone to blackmail." After noting a
couple other "obvious" and "unilluminating" assumptions, Scriven identifies three "illuminating" ones:
So the first serious problem [my emphasis] with the argument is that it
assumes that the individual is not already known to be a homosexual.
[p.171)
So the argument makes the weak assumption [my emphasis] that everyone
would rather pass up a job of this type
114
Michael Burke
than [publicly] acknowledge homosexuality [as a condition of appointment]. And that's something that wasn't
obviously assumed. [po 171]
So the argument, in its general form,
does rest upon an assumption ... [namely]
the proportion of homosexuals in communities which would be powerful
enough to have them removed from their
jobs is greater than the proportion of
heterosexuals in communities which
would react just as unfavorably to heterosexual "misdemeanors." And it isn't
at all clear that the reverse isn't the
case ... That's an interesting discovery
about the argument and a real weakness in it. [p.172, my emphasis]
What Scriven seems to be after,
when analyzing an argument, are the
most implausible propositions that the
argument tru Iy "assumes" or
depends on." (Scriven uses these terms
interchangeably.) The aim is to test
the argument by seeing whether it
can be shown to depend on implausible
assumptions. Of course, this raises,
urgently, the first of the two questions
I distinguished earlier: How can we
tell whether a given inference assumes
a given proposition? Unfortunately,
Scriven never answers this question.
The answer suggested by his examples
is something like this: An inference
/I
assumes (depends on/depends on the
assumption of) proposition A (however
plausible or implausible A may beand however weak or strong) just in
case the inference is justified only if
A is true.
Now I have no objection to Scriven's
method of evaluating arguments. My
objection is to his characterization of
the method. I wi II argue that Scriven's
procedure, contrary to its billing, is
not one of identifying and evaluating
missing premises, but rather one of
evaluating inferences without identifying missing premises. Scriven himself makes the distinction. Near the
beginning of his discussion, he writes
[po 81]:
... this illustrates a very general principle
about argument analysiS. You can either
leave the argument the way you find it,
in which case a good deal of your criti-
cism will be criticism of the inferences
in it; or you can patch it up by adding
some assumptions on which it is obviously depending, in which case the inferences will be pretty satisfactory, but
the assumption will now come under
fire.
Now as Scriven himself remarks
[po 170, p. 171], the "assumptions"
he identifies in the homosexual argument are not ones on which the argument is "obviously depending." The
obvious candidates had been spurned
as "at too trivial a level" [po 169].
So one might expect Scriven not to
conceive the three assumptions" as
unstated parts of the argument, but
rather to conceive their denials as premises of his criticism of the inference.
Now despite the scare quotes I used
in the last paragraph, I do agree that
there is a sense of 'assumes' in which
the homosexual argument does indeed
assume (or might be argued to assume)
the three implausible propositions
Scriven says it assumes. This is the
sense in which an argument may be
said to assume any unstated proposition on which its inference depends,
meaning any proposition which needs
to be true, if the inference is to be justified. But is just any such proposition
a "needed assumption," in the sense
intended by Ennis, Hitchcock, and
Schwartz? No. A needed assumption,
in this latter sense, is one which needs
to be added, or (better) needs to be a
premise of the argument, if the argument is to be as strong as it can. That
is it needs to be included among the
p;opositions from which the conclusion is inferred. Now there are infinitely
many propositions which need to be
true if an inference is to be justified.
(Examples: that there is at least one
inference; that there is at least one
justified inference; that there is at least
one thing that is either an inference
or a polar bear.) Very few of them wi II
be needed to serve as supplements
to the stated premises. That is, very
few of them are needed assumptions,
in any sense in which a needed assumption is a needed premise. And Scriven
has given no reason for thinking that
II
Unstated Premises
the homosexual argument, if it is to
be as strong as it can be, needs to contain as premises the three implausible
assumptions which do (arguably) need
to be true if its inferences are to be
justified.
In short, we should distinguish (1)
used unstated assumptions, i.e., the
unstated propositions actually assumed
by the arguer, (2) needed unstated
assumptions, i.e., the unstated propositions that need to be included among
the propositions from which the conclusion is inferred, if the argument is
to be as stong as possible, and (3) the
unstated assumptions on which the
argument depends, i.e., the unstated
propositions which need to be true,
if the argument's inference is to be
justified. Our concern in this paper is
with unstated assumptions of the
second sort. In chapters titled "Unstated Premises," authors of informal
logic texts generally are concerned
with assumptions of the second or
first sort. Only these assumptions are
properly considered parts of the argument or premises.[3] Searching for the
most implausible assumptions of the
third sort is best conceived as evaluating the inference without first identifying missing parts -a procedure that
may often be eminently suitable.
In sum: I suggest that we take
Scriven not to be offering criteria for
unstated assumptions, in any sense in
which an unstated assumption is a
premise. What Scriven demonstrates
is a method of evaluating inferences
without first identifying missing parts.
The method is that of seeing whether
the inferences can be shown to depend
on implausible propositions. (The linguistic acceptability here of "assumptions" is what can mislead one into
thinking that Scriven is looking for the
same sort of thing as Ennis, Schwartz,
and Hitchcock.) To complete his account, Scriven needs to say what it is
for an inference to I I depend on" a
proposition. (I have offered a suggestion.) He may also wish to draw upon
his neglected weakness, plausibility,
unobjectionability, and obviousness
criteria for an account of used or need-
115
ed unstated premises. (As will soon be
clear, I don't consider this urgent.)
When Scriven deals separately with
(1) used unstated assumptions (2)
needed unstated assumptions, (3)
the assumptions on which an inference
depends, and (4) those of the latter
which are worth identifying as such,
much of the "tension" will be relieved.[4]
I
Section V: Outline of a Policy
Finally I will outline my own ideas
on unstated premises. These ideas are
meant to apply when our aim is to determine whether the stated premises
provide the basis for an acceptable
argument for the conclusion, not when
our aim is to identify the argument intended by the arguer.
As noted by Scriven and others,
it is always possible to evaluate an
argument without first identifying unstated premises. We can instead
evaluate the stated premises and the
inferences. We can evaluate the inferences by seeing, a la Scriven,
whether we can think of any implausible propositions on which the inferences depend-in the sense of
"depend" identified in the preceding
section. Why, then, might we wish to
search for unstated premises?
The answer, I think, is supposed
to be something like this. If we can
find unstated propositions which (1)
satisfy preservation, (2) suffice to
validate, or at least to justify, the inference, and (3) are themselves acceptable, then we will have found reason
for judging the original inference
acceptable. If, on the other hand, we
can find no such propositions, and if
we find a proposition which we have
reason to reject but which we can show
to be needed by the argument, in the
sense that this proposition must be
regarded as an unstated premise if
the argument is to be as strong as it
can be, we will then be able to criticize the argument by giving our reason
for rejecting this proposition.
My view, which I will defend below,
I
116
Michael Burke
is that only when we ourselves accept
a proposition is it useful to identify it
as an unstated premise. If we find no
acceptable candidate satisfying sufficiency and preservation (the two uncontroversial criteria discussed earlier),
then we should evaluate the inference
without identifying unstated premises.
If we search for an acceptable set
of propositions satisfying sufficiency
and preservation, one of three things
will happen: (1) We find just one;
(2) we find more than one; (3) we find
none.
(1) If we find just one acceptable set
of propositions (satisfying sufficiency
and preservation), then we may identify these propositions as unstated
premises. The reason for doing so is to
make explicit (for ourselves or for an
audience) why we accept the original
inference. If we think others may have
doubts about the identified propositions, we may wish to give our reasons
for accepting them. There is more point
in doing this, of course, when we find
the rest of the argument acceptable
aswell.
(2) If we find more than one acceptable set (satisfying sufficiency and
preservation), then we should be
guided by whatever further criteria
guide our choice of stated premises
when we are constructing an argument.
(In effect, we are constructing an argument-an argument for accepting the
original inference.) Evidently there is
little felt need for the identification of
these criteria, and I will be content to
suggest a couple of possible considerations: (a) and (b).
(a) If our choice has narrowed to
candidates A and B, and if B is more
general than A, and if our only reasons
for accepting A are our reasons for
accepting B, then probably we should
pick B. Otherwise we may mislead
our audience concerning our reasons
for accepting A. To assert that jogging
is dangerous for tall women with blue
eyes is to imply that one's evidence
provides significantly less support for
various wider generalizations.
(b) A weaker candidate, especially
the reiterative candidate, will some-
times have less initial plausibility than
some stronger candidate. (In the case
of the intentionally mystifying enthymemes of Sherlock Holmes, the
plausibility of the reiterative candidate
is initially nil.) When the stronger candidate is more plausible initially, and
about as plausible finally, adding the
stronger may be dialectically more efficient. We may avoid having to support
the added premise.
(3) If we find no acceptable proposition (satisfying sufficiency and preservation), I believe we should evaluate
the argument without identifying unstated premises. Actually, I would
allow one trivial exception: We may add
the reiterative candidate. This is always unobjectionable, since the acceptability of the reiterative candidate
is a necessary condition of the acceptability of the argument, but never useful, in regard to facilitating evaluation,
since it is never easier to evaluate the
reiterative candidate than to evaluate
the corresponding inference.
I will now explain why I believe we
should add no proposition unacceptable
to us other than the reiterative candidate. My procedure will be to compare
adding the reiterative candidate, A,
with adding some other candidate,
B (when we accept neither A nor B).
This will be verbally less cumbersome
than comparing adding nothing with
adding some condidate other than the
reiterative one. And, of course, adding
and evaluating the reiterative candidate
is equivalent to adding nothing and
evaluating the inference. But since the
latter strategy is one step shorter, it
is the latter that should be simpler to
employ, although it is the former that
will be easier for me to talk about.
(a) Suppose, then, we are deciding
whether to add the reiterative candidate, A, or some stronger candidate,
B. And suppose that we accept neither
A nor B. Now either we have a stronger
criticism of B than we have of A, or we
do not.
(i) If we do not, then, of course, we
should add and criticize A. We will then
have criticized the weakest assumption
capable of validating the argument.
Unstated Premises
If we added and criticized B, we would
only have to go on to discuss whether
we might accept some weaker, though
still validating proposition. Of course,
it may be that we will criticize A by
criticizing B-and arguing, asserting,
or at least implying that there is no
other reason for accepting A. Even if
so, when our criticism of A is as strong
as our criticism of B, we will want to
make clear that we are attributing to
the argument only the weakest validating assumption and that the point
of our criticism of B is to challenge the
only support for that assumption.
(ii) If, on the other hand, we do have
a stronger criticism of B than of A,
then our aim of evaluating the strongest
argument that can be based on the
stated premises dictates that A be
identified as the unstated premise. If,
for example, we can refute B but not
A, and if we can claim no more than
that B is the only apparent reason for
accepting A, our assessment should
be not that the argument depends on
an unstated premise, B, of demonstrable falsity, but rather that it depends
on an unstated premise, A, for which
there is no apparent justification.
(b) Suppose, finally, that the two
unacceptable finalists are the reiterative candidate, A, and a non-validating
candidate, B. If we have a stronger
criticism of B than of A, then since A
is all that the argument needs, we
should add and criticize A. If our
stronger criticism is of A, then since
the acceptability of A is a necessary
condition of the acceptability of the
argument, again we should add and
criticize A.
In summary: When our purpose is
to determine whether stated premises
provide the basis for an acceptable
argument for a given conclusion, I
recommend the following policy. If
there are unstated propositions satisfying preservation and sufficiency
that we ourselves accept, we may identify them as unstated premises. The
reason for doing so is to make explicit
(for ourselves or for an audience) why
we accept the original inference. If
we find two or more acceptable sets of
117
propositions
(satisfying
sufficiency
and preservation), then we should
choose on the basis of whatever further criteria guide our choice of stated
premises when we are constructing
an argument. (In effect, we are constructing an argument, an argument for
accepting the original inference.) In
part 2b of this section, I indicated the
basis on which we may often reject the
reiterative candidate in favor of one of
its (also acceptable) competitors. If we
find no acceptable candidate (satisfying
sufficiency and preservation), I believe
we should evaluate the inference without identifying unstated premises (or,
which amounts to the same thing, add
and evaluate the reiterative premise).
My reasons were given in part 3 of this
section.
To conclude: In the five sections of
this paper, each of which concludes
with a summary, I have criticized the
unstated-premise policies of Ennis,
Hitchcock, Schwartz, and Scriven; and I
have outlined an alternative.[5]
Notes
[1] Van Eemeren and Grootendorst
have suggested that our target
should be the unstated propositions
to which the speaker is committed
by his argumentative speech-act.
Viewing unstated premises as
Gricean conversational implicatures, they write: "It is not what the
speaker himself may think he has
left unexpressed, but what he can
be held to by the listener, that must
count" [po 220]. Now attention to
conversational implicatures may
yield important evidence of the
arguer's intentions. And if we are
able to engage the arguer in dialogue, pointing out these conversational implicatures may help elicit
the arguer's true intentions. But
unless we are engaging in rhetoric,
or else in an academic exercise,
our ultimate goal will be to identify
and evaluate either the intended
argument or the strongest argu-
118
[2]
[3]
Michael Burke
ment, not the argument to which
the speaker, regardless of his intentions, has lIcommitted" himself.
His frequently repeated "strong
enough, but no stronger than necessary" formula [po 85] doesn't
help here. The reiterative candidate
is counted as an assumption
[po 163], so it must be "strong
enough" to be an assumption. But
any stronger proposition would
then be "stronger than necessary"
to be an assumption. Yet some
stronger propositions are so counted.
Although Scriven explicitly equates
"unstated
assumptions"
and
"Missing premises" [po 179], he
never refers to the three assumptions" of the homosexual argument
as premises. Indeed, in none of
his examples does Scriven use
"missing premise" when referring
to a patently untenable I I unstated
assumption. "
I am charging Scriven's account
with much confusion. I want to add
that I do find it valuable for its insights and illustrations, for its
subtlety, and for distinctions made,
if not observed.
I am indebted to one of this journal's referees, Professor Robert
Ennis, for trenchant criticisms of
an earlier draft.
/I
[4]
[5]
references
Ennis, Robert H., "Identifying Implicit
Assumptions," Synthese 51: 1,
April 1982, pp. 61-86.
Hitchcock, David, Critical Thinking:
a Guide to Evaluating Information,
Methuen
Publications,
Toronto,
1983.
Johnson, Ralph H. /'Charity Begins at
Home ," Informal logic Newsletter
3:3, June 1981, pp. 4-9.
Schwartz, Thomas, The Art of logical
Reasoning, Random House, New
York, 1980.
Schwartz, Thomas, "Logic as a Liberal
Art," Teaching Philosophy 4: 3 & 4,
July/October 1981, pp. 231-247.
Scriven, Michael, Reasoning, McGrawHill Book Company, New York, 1976.
van Eemeren, Frans H. and Grootendorst, Rob, "Unexpressed Premises:
Part II," Journal of the American
Forensic Association 19: 4, Spring
1983, pp. 215-225.
Professor Michael Burke, Department
of Philosophy, Indiana University,
425 Agnes Street, Indianapolis, Indiana 46202